Howcome HIV doesn't spread by kissing?

HIV is an infectous desease that spreads via sexual contact (as well as other ways). Sexual contact would mean contact between mucous membranes. Isn't the inside of the mouth a mucous membrane as well? Is it simply because there is less friction between the membranes when kissing, than during intercourse, that makes kissing "safe"? Is it really safe?

As Monique said, the viral load in saliva is low and it is unlikely that viruses will infected cells. The problem is if the saliva is contaminated with blood or semen. Then the viruse load could be sufficient to start an infection.

But while we're on the subject, does anyone know why HIV can't be spread by mesquito bites?

Because the blood goes into the mosquito digestive system and it is destroyed. Also, the virus that resist the digestion do not travel back to the saliva gland like other virus transmitted by mosquito.

The only possibility would be if a mosquito did not finish his blood meal from an HIV person and starts immediatly to feed on an non-HIV person. The proboscis may still have blood on it. If the mosquito has finish his blood meal it is unlikely that HIV will be transmitted because it usually rest after a blood meal. The rest is enough to kill the virus.

Because the blood goes into the mosquito digestive system and it is destroyed. Also, the virus that resist the digestion do not travel back to the saliva gland like other virus transmitted by mosquito.

The only possibility would be if a mosquito did not finish his blood meal from an HIV person and starts immediatly to feed on an non-HIV person. The proboscis may still have blood on it. If the mosquito has finish his blood meal it is unlikely that HIV will be transmitted because it usually rest after a blood meal. The rest is enough to kill the virus.

This is exactly what I was thinking of. There is no way all of the blood from one host is gone from the proboscis when the mosquito moves to the next host, is it? I can see how the situation is any different from intravenous drug users sharing a needle. They use a needle to push fluid into the vein, rather than drop it out, and even then some blood remains on the needle and gets transferred to the next user.

The HIV virus doesn't survive all that long exposed to air, so any virus on the outside of the mosquito's proboscis wouldn't only survive a short time. However, mosquitoes also don't stick their proboscis all the way into you like a needle, they instead secrete some saliva that prevents your blood from clotting (that's what makes mosquito bites itch) and then drink the blood that way. It's probably also a matter of dose. A mosquito proboscis is pretty tiny compared to a hypodermic needle, the mosquito's objective is to extract blood, not inject it into you like a needle would, and a hypodermic needle shared by drug users would also have blood inside it that is then injected into the next user.

The same with kissing. Nobody can say there is absolutely NO risk to kissing someone infected with HIV, they can say there is almost no risk. Not much, if any virus gets into the saliva and lasts long, but tiny cuts in the mouth may change that. Actually, during sexual intercourse, the friction can cause tiny tiny tears in the mucous membranes, and IIRC, it's through those tears that the virus is thought to be transmitted most efficiently (these are not tears anyone would notice, more like microscopic abrasions, just enough for the virus to get in).